This invention relates to photovoltaic devices and a method of making the same wherein the devices are formed from layers of amorphous semiconductor alloys in which the bandgaps thereof can be graded imperceptibly. The devices are formed from layers of amorphous silicon alloys which have different bandgaps. One advantage of this graded material approach is that the devices have improved photoresponsive characteristics for providing enhanced short circuit currents. Another advantage, as explained hereinafter, is that the improved photoresponsive characteristics of fluorinated amorphous silicon alloys can be more fully realized in photovoltaic devices by practicing the present invention. The invention has its most important application in making improved amorphous silicon alloy photovoltaic devices of the p-i-n configuration, either as single cells or multiple cells comprising a plurality of single cell units.
It is well known that grading single phase cyrstalline silicon is an exceptionally difficult if not impossible task since the different band gaps and the lattice mismatch introduce insurmountable problems. This is especially so when indirect band gap materials are utilized and thick materials are required. In amorphous materials, for example, it is not sufficient to list a large array of different amorphous layers that could be graded since in a photovoltaic cell the important parameter is that the intrinsic material has a very low density of states. There are two amorphous materials with low density of states, one is a material deposited from silane and the other is a material that contains fluorine (see for example U.S. Pat. No. 4,217,374 which issued in the names of Stanford R. Ovshinsky and Masatsugu Isu on Aug. 12, 1980 for Amorphous Semiconductors Equivalent to Crystalline Semiconductors and U.S. Pat. No. 4,226,898 which issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980 under the same title). The fluorinated material is a superior material in every way and it is the bulk intrinsic properties of this material which makes it the basis for a superior amorphous photovoltaic cell. However, fluorine can also be an etchant, which can be a vice or virtue (see for example, U.S. patent application Ser. No. 331,259, filed concurrently herewith). In certain cases, its negative aspects are that by attacking other layers, it can contaminate the intrinsic material by incorporating in them elements such as boron or phosphorus. In order to prevent this problem and to make an improved cell, there is described herein an invention in which the fluorine is utilized for its basic superior qualities and in which a thin layer of material not containing fluorine is utilized to interface a highly doped contact layer which normally would be reactive to fluorine, and the resultant combination brings several unique advantages. There is less potential contamination of the dopants in the fluorinated material, and the non-fluorinated material such as that deposited from silane, can be selected to have a smaller band gap. This increases the current output from such a multi-layer device. Since there is no lattice, there is no lattice mismatch, and both band gaps can be matched without introducing any other materials that might increase the density of states and therefore degrade device performance. The resulting device has sharp junctions, and other advantages can be foreseen.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells for space applications. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure cyrstalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other defect problems.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous silicon alloys can eliminate these crystal silicon disadvantages. An amorphous silicon alloy has an otpical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon alloys can be made faster, easier and in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. B. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, (1975), toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping cyrstalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
The incorporation of hydrogen in the above method however has limitations based upon the fixed ratio of hydrogen to silicon in silane, and various Si:H bonding configurations which introduce new antibonding states. Therefore, there are basic limitations in reducing the density of localized states in these materials.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, which issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, which issued in the name of Stanford R. Ovshinsky and Masatsugu Izu, on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor alloy to substantially reduce the density of localized states therein. Activated fluorine especially readily diffuses into the bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine also combines in a preferable manner with silicon and hydrogen, utilizing the hydrogen in a more desirable manner, since hydrogen has several bonding options. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect status in the band gap as well as in the material itself. Therefore, fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its high reactivity, specificity in chemical bonding, and high electronegativity.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
Amorphous silicon alloys containing fluorine alone or with hydrogen have thus demonstrated greatly improved characteristics for photovoltaic applications as compared to amorphous silicon alloys containing just hydrogen alone as a density of states reducing element. However, it has been observed that when amorphous silicon alloys containing fluorine are deposited on doped amorphous silicon alloy layers, as by the glow discharge deposition of silicon tetrafluoride and hydrogen for example, small amounts of the material including the dopant of the previously deposited doped layer can be removed and redeposited with the new alloy layer. This can create a relatively thin region of material between the doped layer and the intrinsic layer having multiple phases, possible potential gradients, and a high density of states, all of which can adversely affect the electrical and photo-responsive characteristics of the photovoltaic devices in which they are employed. It is believed that the starting materials for the silicon-fluorine alloy, when decomposed in the glow discharge plasma, become etchants of the previously deposited layer and remove the small amounts of the material by etching the same. This etching continues for only a short period of time until a substantially pure amorphous silicon-fluorine alloy begin to be deposited, resulting in the relatively thin region of deleterious material between the two layers.
The foregoing becomes especially important in the fabrication of photovoltaic devices of the p-i-n configuration. Devices of this type require the deposition of a first doped layer followed by the deposition of an intrinsic layer. If the superior characteristics of amorphous silicon-fluorine alloys are to be fully achieved, it is necessary to deposit the amorphous silicon-fluorine-hydrogen intrinsic alloys without removing and redepositing the material of the doped layer.
Applicants have discovered a new device structure and method of making the same which allows all of the advantages of amorphous silicon-fluorine alloys to be realized in a photovoltaic device of, for example, the p-i-n configuration without forming the previously referred to deleterious region between the first doped layer and the intrinsic layer. Further, the devices and method of the present invention may also be utilized to advantage in multiple cell structures having a plurality of single cell units. The invention also allows the adjusting of the band gaps of the intrinsic and doped layers to maximize the photoresponsive characteristics thereof and the production of amorphous semiconductor single and multiple cell photovoltaic devices of enhanced current capability and efficiency.